1. Field of the Invention
The present invention relates to the completion and stimulation of oil wells, gas wells, injection wells, water wells and similar boreholes.
2. Brief Description of the Prior Art
After drilling a borehole into a subterranean formation, tubing or casing is normally placed in the borehole to provide a conduit through which fluids may flow. The tubing or casing must be perforated so that fluids can be transported to or from the producing zones through the well bore.
Many times in completing wells, the flow of fluids into or out of the formation must be prevented. One instance is when a highly permeable zone of the formation is encountered and much of the fluid pumped into the well for drilling, completion, or stimulation purposes, leaks off into the formation through the highly permeable region. Another instance is when the well is shut-in, i.e., remains dormant for some period of time. In this situation, pressure in the well bore must be equalized with the formation pressure so that formation fluids will not enter the well bore. Heavy brines, defined as aqueous liquids nearly saturated with calcium chloride (CaCl.sub.2), calcium bromide (CaBr.sub.2), zinc bromide (ZnBr.sub.2) or mixtures of these salts, are normally placed in the well bore to equalize formation pressure.
For the brine to equalize pressure, it must be contained within the well bore. Perforations in the tubing or casing (or the highly permeable zones of the formation as the case may be) must be sealed off or blocked. As a result, many compositions have been developed to perform this blocking function. Examples of these compositions, known in the art as temporary blocking gels, include crosslinked polysaccharides and crosslinked synthetic polymers. The term `gel` is used hereafter to mean crosslinked polymer solutions.
Gels are also used in stimulating subterranean formations. Formations are stimulated by injecting fluids into the well bore at sufficient rates and pressures to produce fractures in the formation. When gels are used as fracturing fluids, they must retain viscosity and structure at formation temperatures to produce the desired fracture. Once the fracture is formed, the gel must "break", that is, the viscosity must be sufficiently reduced so that the fluid can be pumped or produced from the formation through the well bore.
For temporary blocking gels, viscosity and structure must remain in tack for extended time periods (from hours to weeks) depending on the particular well condition. The gels must also predictably degrade over time so that the well can be returned to production or to perform other treatments on the well. The blocking gels must break completely so that the perforations and nearby formation are free of gel residue when the broken gel is removed from the well.
Various chemicals have been used to break these highly viscoelastic gels, for example, persulfates, peroxides and acids. However, these compositions have had limited utility in that the gel breaks either uncontrollably (i.e., too quickly or not at all) or only at particular temperature ranges. Breaking gels in formations with low temperatures, used hereafter to mean formation temperatures less than 225.degree. F., is particularly difficult using conventional breakers.
The present invention overcomes the deficiencies of conventional breakers by providing a new method of predictable, controlled breaking of polysaccharide solutions and gels used in well completion and stimulation operations.